The invention proceeds from known electrodes consisting at least of an electrically conducting substrate based on a valve metal and an electrocatalytically active coating of a noble metal oxide or noble metal oxide mixture and titanium oxide.
Prior art chlorine production utilizes electrode coatings consisting of ruthenium-titanium oxide mixtures (e.g. dimensionally stable anodes, DSA™). The composition of the coating, i.e. the ratio of ruthenium to titanium oxide, is the decisive factor in that it decides electrocatalytic activity. Commercial DSA™ consist of 30 mol % RuO2 and 70 mol % TiO2. As described in J. Electrochem, Soc. 124, 500 (1977), the coating is composed of a main phase consisting of a TiO2-ruthenium oxide solid solution of rutile structure, and of secondary phases of pure ruthenium oxide and a pure anatase phase. U.S. Pat. No. 3,562,008 describes a coating of predominantly amorphous titanium oxide with crystalline noble metal oxide or noble metal. Furthermore, as described in Russ. J. Electrochem, 38, 583 (2002) and Mat. Chem. and Phys. 22, 203 (1989), hydrated ruthenium oxide can be present alongside amorphous, hydrated oxide phases. The printed publications Electrochimica Acta 40, 817 (1995) and Electrochimica Acta 48, 1885 (2003) show that RuO2—TiO2 coatings produced by means of a thermal decomposition process result in a product which has a structural short range order. These heterogeneously constructed layers contain microclusters of RuO2 and TiO2 domains, which are randomly distributed in the layer. The electronic conductivity of these layers can be described in terms of percolation theory (Journal of Solid State Chemistry 52, 22 (1984)). The theory explains the conductivity of very finely divided and conductive particles (RuO2 domains) in an insulating matrix of TiO2 domains. According to this theory, the electronic properties are determined by the homogeneity of the mixed oxide. Any activity enhancement and any improvement in the useful life of the coating is only achievable when the active component RuO2 can be homogeneously distributed on a molecular scale. Such a distribution of RuO2 in a TiO2 matrix can be achieved, as described in Journal of Sol-Gel Science and Technology 29, 81 (2004) and Colloids and Surface A 157, 269 (1999), by the use of a sol-gel process. In this sol-gel process, the components become distributed at a molecular level as a result of the hydrolysis of suitable precursor substances. The advantages of the sol-gel process are:                1. The reaction at low temperatures makes it possible to produce very small nanostructures.        2. The hydrolysis of the starting materials gives rise to products (RuO2—TiO2) which are divided homogeneously and at a molecular level, and are formed by chemical interactions (e.g. bonds). The homogeneous distribution of the resulting oxides in the electrode coating gives rise to electronic paths of conductance which ensure optimum flow of current.        
In contrast to coatings produced via thermal decomposition of labile starting materials, layers produced by the sol-gel process exhibit better electronic and mechanical properties due to the homogeneity of the mixing operation. This additionally provides higher stability to the layers. As stated in Journal of Electroanalytical Chemistry 579, 67 (2005), samples produced via sol-gel processes show that the impedance of the samples rises less in the course of chlorine evolution than that of samples produced via thermal decomposition. This observation suggests higher activity for the samples produced via sol-gel processes. One disadvantage of the sol-gel route is the limited scope for varying the phase composition in the binary RuO2TiO2 layer. Phase composition can be controlled to a small extent by varying the pH, the starting composition and the sintering temperature. These possibilities are described in Materials Chemistry and Physics 110, 256 (2008), Journal of the European Ceramic Society 27, 2369 (2007), Journal of Thermal Analysis and calorimetry 60, 699 (2000), Chem. Mater. 12, 923 (2000) and J. Sol-Gel. Sci, Techn 39, 211 (2006). The phase formation behaviour between RuO2—TiO2 is described in Journal of the Electrochemical Society 124, 500 (1977). TiO2 occurs in two polymorphic phases, rutile and anatase. While anatase is stable at low temperatures, rutile occurs at high temperatures only. The phases can be converted into each other via thermal treatment. A further possibility of conversion is the addition of a second component in the form of a dopant. This dopant adds onto the TiO2 structure and thereby influences the coordination which leads to the formation of a homogeneous rutile or anatase phase. By the very good lattice matching between tetragonal RuO2 and tetragonal TiO2 (rutile), the formation of the latter is favoured. Therefore, conventional coatings have a main constituent consisting of a solid mixture of RuO2/TiO2 with corresponding tetragonal structure. Depending on the method of production, layers having an RuO2 content of 20-40 mol % may contain small proportions of anatase phase. The thermodynamic stability of the structure, i.e. the bonding behaviour of the MO6 octahedra of Ru and Ti, depends on the free surface energy of the nanoparticles, which is influenced by the surface chemistry (oxide and hydroxide formation, water adsorption) (Nano Letter 5, 1261 (2005)). In general, the thermally induced crystallization of amorphous phases under oxidizing conditions leads to a coating structure having a rutile phase as main proportion. This process is due to oxygen surface adsorption. Hitherto no electrocatalytically active coating systems having a main proportion of anatase phase are known.
Surprisingly, it was found, coatings having an increased anatase fraction exhibit an increased electrocatalytic activity for chlorine evolution in comparison with layers based on rutile structure. This invention accordingly has for its object to produce electrocatalytically active coatings having a main proportion of anatase phase.